Such a alias will not work as every website runs under ist own Linux user, thats why we did not implement it in ISPConfig. If you want your client to be able to access a site before the domain is registered, you can either add a subdomain to this website of a already registered domain or your client can set his domain in hosts hosts file:

Add a wildcard dns record for tecatest.com.ar that points to your server. Then add a aliasdomain alte.tecatest.com.ar to the website alte.com.ar and your customer can access his website with alte.tecatest.com.ar